Solar thermal conversion involves redirecting solar radiation that strikes a large area such that the redirected solar radiation strikes a smaller thermal receiver. The redirection of solar light is typically accomplished with large numbers of reflectors. The thermal receiver has a target area that is considerably smaller than the overall solar collection area of the reflectors. This results in the area of the thermal target (thermal receiver and heat exchanger) and, therefore, the black body radiation thermal loss, being reduced (thus increasing efficiency).
Since the sun is in constant movement throughout the day with respect to any one point on the Earth's surface, a tracking mechanism may be provided to maintain the concentration effect during daylight hours. There are two basic schemes of tracking mechanism: movable target schemes and fixed target schemes. Parabolic dish and parabolic trough are 2-dimensional and 1-dimensional examples, respectively, of the first type, and tower and linear Fresnel are 2-dimensional and 1-dimensional examples, respectively, of the second type.
Fixed target scheme solar plants typically utilize multiple reflectors, each supported by an individual tracking mechanism (also referred to as a heliostat—the term “heliostat” may also be used, in some contexts, to refer to any concentrator-type solar power system in which the target remains stationary and the sunbeams are steered onto the target by movable reflectors), while keeping the receiver (target) fixed atop a tower. For the last four decades, fixed-target solar thermal plants have largely focused on systems that utilize a single, large target tower and a large field of sun-tracking reflectors distributed across an annular segment of 90° or more of arc. For example, the Solar One pilot plant built in the 1970s featured 1818 identical reflectors distributed across an annular area measuring nearly 0.5 mi across and included a single 90 meter tower. Subsequent tower solar concentrators include Solar Two (adding 108 additional, larger reflectors to the outer perimeter of the annular area closest to the equator of Solar One), SPP-5 (Ukraine, 1600 reflectors distributed across an annular area), Planta Solar 10 and Planta Solar 20 (624 reflectors and 1255 reflectors, respectively, distributed across approximately 90° to 180° angular segment on the opposite side of the respective towers from the equator). The Ivanpah cluster of solar thermal plants, which started generating power in 2013 and 2014, features three towers each 485 ft high with over 173,500 heliostats divided amongst them (Unit 1 of Ivanpah has 53,527 of the heliostats, while Units 2 and 3 of Ivanpah have 60,000 heliostats each).
Since the optical focal length of each reflector in a conventional tower scheme changes with rotation angle of the reflector when the incoming light is not normal to the focal plane of the reflector, the concentration ratio of each individual reflector relative to the target is kept relatively low for conventional solar thermal tower systems, i.e., between 1 and 3. The concentration ratio is the ratio of the total reflector area for a reflector divided by the target face area of the receiver. A 1:1 ratio, for example, results from a planar mirror having the same reflective surface area and shape as the target face area (in practice, such a reflector may still require a slight degree of curvature to compensate for the angular dispersion of a sunbeam over distance—the amount of curvature is dependent on the distance between the target face and the reflector). A higher concentration ratio may be achieved by utilizing concave reflectors; however, concave reflector units suffer large drop-offs in efficiency when reflecting light along directions other than their focal direction. In order to compensate for the natural dispersion angle of sunlight, conventional tower systems typically utilize large, substantially planar reflectors (with mild degrees of curvature) that have individual solar concentration ratios of approximately 3:1 or less (to avoid overspill of reflected light past the target and to reduce manufacturing costs); current conventional tower systems do not exceed solar concentration ratios greater than 3:1 for each tracking reflector due to the efficiency drop-offs associated with such concentration ratios when reflecting light along directions other than their focal direction.
To achieve the large concentration ratio needed to compensate for black body radiation loss, conventional tower systems often require hundreds (sometimes more than 1000) of tracking reflectors. Since each tracking reflector needs a tracking mechanism, large reflectors, e.g., 40 m2 up to 120 m2, are typically used to keep the cost down.
In movable target systems, each reflector has its own receiver unit that is fixed in space relative to the reflector. A tracker causes the reflector to track the sun such that the reflector unit always directs sunlight onto the corresponding receiver (which moves with the reflector).
In both the fixed and movable target schemes, a heat transfer liquid may be pumped through the receiver, heated up, e.g., to generate steam, and then routed to a turbine or other power-generation mechanism.
There are great benefits of conventional fixed target systems over conventional movable target systems: 1) trackers for reflectors alone are much cheaper than trackers for reflectors with movable receivers due to reduced acceleration torque (even if such systems are balanced to eliminate torque, the tracker drive motors still have to overcome the inertial effects of the reflector and the cantilevered receiver); 2) fixed target systems do not need to route the heat transfer liquid through the multi-axis rotational joints of the heliostats (which typically requires expensive feed-through devices for the heat transfer fluid) and typically have greatly reduced lengths of thermal fluid piping in the 2-D case; and 3) a much higher overall optical concentration ratio can be achieved on the target face. There are some drawbacks to conventional tower systems as compared with conventional movable target systems, however: 1) average optical cosine loss is large since reflectors and receivers are rarely in-line with the sun; this loss can be as large as 23% in tower solar thermal plants and even larger in linear Fresnel power plants; 2) in the tower solar thermal plant case, in order to keep the solar concentration ratio large and the overall cost low enough, a large reflector field is needed, e.g., often hundreds or thousands of reflectors, which in turn causes the optical attenuation in air to be significant—this is especially detrimental in places where atmospheric turbidity (or particulate concentration in the atmosphere) is large; 3) in the 1-D case, the practical optical concentration ratio is much reduced compared to the case of 1-D trough collectors (with moving targets).
In this disclosure, we propose novel design principles, apparatus and methods to dramatically overcome the drawbacks of tower systems while maintaining the benefits of tower systems.